| Responses of
inferior colliculus neurons to harmonic and mistuned complex tones
Donal G. Sinex, Jennifer Henderson Sabes, and Hongzhe Li Hearing Research, Volume 168, Issues 1-2, June 2002, Pages 150-162 Department of Speech and Hearing Science, Arizona State University, Box
871908, Tempe, AZ 85287-1908, USA Abstract: Responses of inferior colliculus neurons to simplified stimuli that may
engage mechanisms that contribute to auditory scene analysis were obtained.
The stimuli were harmonic complex tones, which are heard by human
listeners as single sounds, and the same tones with one component 'mistuned',
which are heard as two separate sounds. The temporal discharge pattern
elicited by a harmonic complex tone usually resembled the same neuron's
response to a pure tone. In contrast, tones with a mistuned component
elicited responses with distinctive, stereotypical temporal patterns that
were not obviously related to the stimulus waveform. For a particular
stimulus configuration, the discharge pattern was similar across neurons
with different pure-tone frequency selectivity. A computational model
that compared response envelopes across multiple narrow bands successfully
reproduced the stereotypical response patterns elicited by different
stimulus configurations. The results suggest that mistuning created a
temporally synchronous distributed representation of the mistuned component
that could be identified by higher auditory centers in the presence of
the ongoing response produced by the remaining components; this kind of
representation might facilitate the identification of individual sound
sources in complex acoustic environments. Comment: This paper reports a major breakthrough in the research of consonance
and dissonance. Consonance has two components, musical consonance and
sensory consonance. The former is dependent on musical habits, but the
latter is dependent on the physiology of the auditory system. In 1863
Helmholtz suggested that sensory dissonance results from a perception
of roughness, which is due to beat frequencies produced by interacting
spectral components (partials, harmonics). This theory was confirmed and
extended in the 1960s by Plomp and others on the basis of psychoacoustic
experiments. Until now it was uncertain, however, if and where the beat
frequencies that underlie the roughness in sensory dissonance are represented
in the auditory brain. Sinex, Henderson, and Li could now record the
exact beat frequencies in single neurons of the auditory midbrain,
i.e., the central nucleus of the inferior colliculus, in the chinchilla.
For example, the sound components at 1120 Hz and 1250 Hz caused a frequency
component of 130 Hz in the firing response of these neurons, thus exactly
mirroring the acoustical beat frequency. It is interesting that similar
"neuronal beats" are less clear in the first neurons that leave
the inner ear, and in the auditory cortex they do not occur in single
neurons at all. The current theory is that temporal information from
separate, frequency-band-specific, neuronal channels is combined in the
auditory midbrain, resulting in strong and clear neuronal beats there.
Above this nucleus, however, the temporal information is no longer conserved,
but transformed to other codes in many specialized upward neurons. Some
of them apparently register the occurrence of beats in the midbrain and
then contribute to the perception of roughness and dissonance. |